1. Field of the Invention
This invention relates to tissue treatment by intravascular radiation wherein the radiation source is a safe, liquid suspended, short-lived radioisotope. More specifically this invention is well suited to the radiation treatment of blood vessel walls during or subsequent to their treatment by balloon angioplasty in order to delay or eliminate restenosis thereof.
2. Description of the Related Art
Percutaneous transluminal angioplasty (PTA) treatment of the coronary arteries, percutaneous transluminal coronary angioplasty (PTCA), also known as balloon angioplasty, is the predominant treatment for coronary vessel stenosis.
Popularity of the PTCA procedure is attributable to its relatively high success rate and minimal invasiveness as compared with coronary by-pass surgery. However, patients treated by PTCA suffer from a high incidence of restenosis. About 35% of all patients require repeat PTCA procedures or by-pass surgery, with attendant high cost and added patient risk.
Restenosis occurs as a result of injury to the arterial wall during the lumen-opening angioplasty procedure. The injury initiates a repair response in some patients that is characterized by hyperplastic growth of the vascular smooth muscle cells in the region traumatized by the angioplasty. The hyperplasia of smooth muscle cells narrows the lumen that was opened by the angioplasty, thereby necessitating a repeat PTCA or other procedure to alleviate the restenosis.
Studies have indicated that intravascular radiotherapy (IRT) may be used to prevent stenosis following cardiovascular graft procedures or other trauma to the vessel wall. Proper control of the radiation dosage, however, is critical to impair or arrest hyperplasia without causing excessive damage to healthy tissue.
Radiation dose must be accurately and uniformly applied in order to inhibit smooth muscle cell proliferation while sparing other tissues of the vessel wall. All techniques currently being actually explored with respect to human patients utilize solid long lived sources in the form of thin wires or beads. These sources must be advanced through a thin catheter on the order of 100 cm in length and positioned in the section of artery requiring treatment. Such techniques carry substantial drawbacks in both accuracy and uniformity of dose application as well as in patient safety. Typical arteries being treated are often highly asymmetric. Therefore the wall dose can be circumferentially nonuniform as a result of positioning asymmetry in the absence of precise centering of the source within the artery, which is very difficult to accomplish. Furthermore patient safety is compromised by the risk of lodgement of the source within the vascular system. Although infrequent, such incidents can cost the life of the patient and also can lead to radiation safety risks to health care workers undertaking emergency surgery or other recovery measures.
U.S. Pat. No. 5,616,114 describes a method of intravascular radiotherapy utilizing a liquid suspended radioactive source such as P-32 or I-125. Radiation treatment to a blood vessel wall is accomplished in the preferred embodiment through filling of an intravascular thin-walled balloon with such a source so that intimate contact is achieved between source liquid and the vessel wall.
Filling of a balloon with a radioactive solution in such a way as to achieve intimate contact between the radiation source and the target tissue is a desirable solution to the problem of uniform dose delivery. Although animal studies have clearly demonstrated the feasibility of such technique, no human applications are currently being considered because of the lengthy half lives of commercially available radioactive filling solutions such as P-32 (half life=14 days) and I-125 (half life=60 days). Because of these lengthy half lives and the potential for rupture of the balloon with release of contents into the patient's blood, such isotopes are far too risky to be considered.
In such an event these long-lived sources would distribute and stick in critical tissues such as bone marrow and, because they impart radiation for a lengthy period, can cause death of the patient or substantially increased risks of development of cancers such as leukemia. Accordingly, although elaborate means are described in U.S. Pat. No. 5,616,114 for containment of the source liquid, such procedure as there described has not been utilized on human patients because of this grave danger. Also the level of radioactivity in the solution must be limited to a suboptimal level which requires lengthy treatment times causing further patient risk.
Although an IRT procedure performed through the filling of an intravascular thin-walled balloon with a radioactive source appears to have several advantages with respect to uniformity and location of dosing of that tissue in need of treatment, as heretofore proposed as in U.S. Pat. No. 5,616,114, the extreme dangers involved in balloon rupture within a human patient with its invivo release of the radioactive P-32 and/or I-125 source have precluded its practical use in human patients, even on an experimental basis.